A two dimensional dot code is a dot pattern printed under specific rules for reproducing specific information. Nowadays, two dimensional dot codes are often used in teaching materials, for example English teaching materials for children. In these teaching materials, behind the clearly printed main characters, relatively small and inconspicuous two dimensional dot codes are printed as the background of the main characters. A user reading the main characters can simultaneously scan the two dimensional dot codes in the background with an optical reader and thereby input the two dimensional dot codes into a computer or other devices, which decode the two dimensional dot codes and output corresponding information to make a strong impression of the main characters on the user. For instance, a two dimensional dot code read from the background of a main character of the English letter A may result in playback of an audio file containing the pronunciation of the letter A, or playback of a video file related to the letter A, such as one showing an apple falling from a tree.
Generally, a block of two dimensional dot code is composed of boundary dots, direction dots and code dots. To decode a two dimensional dot code, an optical reader is used to input the image of the two dimensional dot code into a decoding device, such as a computer. Each commercial company defines its own encoding rules, and thereby produces unique two dimensional dot codes in terms of positioning method, code dot distribution and code capacity. For instance, Taiwan Pat. No. 581,970 issued to Sonix Technology Co., Ltd. discloses a two dimensional dot code, in which boundary dots (a header group) arranged in an L shape define the range and direction of the two dimensional dot code, so that information represented by the block of two dimensional dot code can be deciphered according to positions of code dots in the block of two dimensional dot code. U.S. Pat. No. 6,548,768 issued to Pettersson et al. teaches a two dimensional dot code having no specific boundary dots. Instead, two code dots having the shortest spacing therebetween is first found out, virtual grid lines are plotted based on the two code dots to further define the dimensions of virtual grid cells, and finally a range of a block of two dimensional dot code is simulated to facilitate decoding. Presently, the maximum code capacity of a two dimensional dot code is 232, made by encoding rules proposed by PixArt Imaging Inc.
Although encoding rules vary from company to company, decoding a two dimensional dot code always begins with determining an angular difference between an image of the two dimensional dot code and a preset direction according to boundary and direction dots because a scanned image of the two dimensional dot code is often skewed. Only when the image of the two dimensional dot code is rotationally corrected can information represented by the two dimensional dot code be determined according to the positions of code dots, before performing the action corresponding to the information. However, calculation of rotation is complicated and susceptible to misjudgment because not only are sine and cosine operations involved, but also the farther a code dot is from the center of rotation, the greater the error.
The present invention provides a decoding method for a two dimensional dot code, suitable for use with any two dimensional dot codes whose boundary dots and direction dots have certain geometric extrapolation and interpolation relations. With this method, positions of code dots in a two dimensional dot code can be rapidly determined without rotational correction so as to decode the two dimensional dot code speedily.